"Safer for kids" isn't a factor here; Apple already provides tools for parents to lock down their children's devices should that be necessary. This would also presumably have an option (or default) to disable third-party apps, in the same way parents can already disable specific App Store and built-in apps.
And let others use the Epic app store for Fortnight.
I value the customer's ability to install programs of their choosing on a phone they purchased. It seems strange that we see it as desirable to concentrate authority over billions of devices this way. Taking a step back, it's sort of a "what could possibly go wrong" scenario.
A single source of application signing will always be abused to entrench dangerous governments and prejudice. It is toxic, and needs to be outlawed.
Opening up the platform to other app stores means they have less control over the experience on their platform it opens more avenues for malware.
If you want a more open platform use Android.